Method and wireless communications system using coordinated transmission and training for interference mitigation

ABSTRACT

A method for interference mitigation in a wireless communication system having multiple transmitters and receivers by introducing transmission time delays between the transmission of signals from the individual transmitters to ensure coherent reception of the signals at a specific point in the coverage area, such as at a center of distribution of the receivers. To further aid in interference mitigation the signals are assigned training patterns chosen to be distinguishable by the receiver and to optimize interference mitigation. The training patterns can be selected based on a feedback parameter, e.g., a measure of the quality of interference mitigation obtained from the receiver. The present method can be used in wireless communication systems which re-use frequencies including TDMA, CDMA, FDMA, OFDMA or other multiplex communication systems using a multiple access method or a combination of such methods.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems and methods of operating such systems to mitigate interferencewith the aid of coordinated transmission and training.

BACKGROUND OF THE INVENTION

Wireless communication systems serving stationary and mobile wirelesssubscribers are rapidly gaining popularity. Numerous system layouts andcommunications protocols have been developed to provide coverage in suchwireless communication systems.

Currently, most wireless systems are broken up into separate coverageareas or cells. Typically, each cell has a base station equipped with anantenna for communicating with mobile or stationary wireless deviceslocated in that cell. A cellular network consists of a number of suchcells spanning the entire coverage area. The network has an assignedfrequency spectrum for supporting communications between the wirelessdevices of subscribers and base stations in its cells. One of theconstraints on a wireless communication systems is the availability offrequency spectrum. Hence, any wireless system has to be efficient inusing its available frequency spectrum.

It is well-known that attenuation suffered by electromagnetic wavepropagation allows wireless systems to re-use the same frequency channelin different cells. The allowable interference level between signalstransmitted in the same frequency channel determines the minimumseparation between cells which can be assigned the same frequencychannel. In other words, frequency channel re-use patterns are dictatedby the amount of Co-Channel Interference (CCI) seen by the receivingunit (either the base station or the wireless subscriber device).

As an example of frequency re-use, FIG. 1 shows a portion or a cluster10 of a typical wireless cellular system with a 7*3 re-use schedule,i.e., spatial channel re-use factor 7 and 3 sectors using differentfrequency channels in each cell 12. In the 7*3 case the availablefrequency spectrum is divided into 21 channels or sub-channels labeledby f₁, f₂, . . . , f₂₁. Frequencies f₁, f₂, f₃ are used in cell 12A,frequencies f₄, f₅, f₆ are used in cell 12B and so on. There is nofrequency re-use within cluster 10.

FIG. 1B shows a system 14 built up of clusters 10. As can be seen, theclosest cell which re-uses the same frequency channel is at least threecells away. This separation ensures that sufficient attenuation isexperienced by the signals emitted in the cells of one cluster beforereaching cells of the next cluster re-using the same frequencies in itscells to not impair communications. The capacity of system 14 isdictated by the bandwidth of the channels and thecarrier-to-interference (C/I) ratio. The sustainable re-use structure,therefore, decides the spectral efficiency of the system which ismeasured in the amount of information transmitted per unit frequency percell, commonly measured in bps/Hz/cell.

Clearly, high spectral efficiency is a desirable system characteristic.By reducing CCI the C/I ratio can be improved and the spectralefficiency increased. Specifically, improved C/I ratio yields higher perlink bit rates, enables more aggressive frequency re-use structures(closer spacing between cells re-using the same frequency channels) andincreases the coverage of the system.

It is known in the communication art that receiving stations equippedwith antenna arrays, rather than single antennas, can improve receiverperformance. Antenna arrays can both reduce multipath fading of thedesired signal and suppress interfering signals or CCI. Such arrays canconsequently increase both the range and capacity of wireless systems.This is true for instance of wireless cellular telephone and othermobile systems.

In mobile systems, a variety of factors cause signal corruption. Theseinclude interference from other cellular users within or near a givencell. Another source of signal degradation is multipath fading, in whichthe received amplitude and phase of a source varies over time. Thefading rate can reach as much as 200 Hz for a mobile user traveling at60 mph at PCS frequencies of about 1.9 GHz. In such environments, theproblem is to cleanly extract the signal of the user being tracked fromthe collection of received noise, CCI, and desired signal portionssummed at the antennas of the array.

In Fixed Wireless Access (FWA) systems, e.g., where the receiver remainsstationary, signal fading rate is less than in mobile systems. In thiscase, the channel coherence time or the time during which the channelestimate remains stable is longer since the receiver does not move.Still, over time, channel coherence will be lost in FWA systems as well.

Antenna arrays enable the system designer to increase the total receivedsignal power, which makes the extraction of the desired signal easier.Signal recovery techniques using adaptive antenna arrays are describedin detail, e.g., in the handbook of Theodore S. Rappaport, SmartAntennas, Adaptive Arrays, Algorithms, & Wireless Position Location; andPaulraj, A. J et al., “Space-Time Processing for WirelessCommunications”, IEEE Signal Processing Magazine, November 1997, pp.49-83.

Some of the techniques for increasing total received signal power useweighting factors to multiply the signal recovered at each antenna ofthe array prior to summing the weighted signals. Given that antennaarrays offer recognized, advantages including greater total receivedsignal power, a key issue is the optimal calculation of the weightingfactors used in the array. Different approaches to weight generationhave been presented in the art.

If the channels of the desired and interfering signals are known, theweight generation technique that maximizes thesignal-to-interference-plus-noise ratio (SINR), as well as minimizes themean squared error (MMSE) between the output signal and the desiredoutput signal, is the well-known Weiner-Hopf equation:

w=[R_(xx)]⁻¹r_(xd),

where r_(xd) denotes the crosscorrelation of the received signal vectorx with the desired signal, given by:

r_(xd)=E[x*d],

where d is the desired signal, and R_(xx) is the received signalcorrelation matrix, which in turn is defined as:

R_(xx)=E[x*x^(T)],

where the superscript * denotes complex conjugate and T denotestranspose.

Of course, this technique, also known as the beamforming approach, isonly one of many. Other prior art techniques include joint detection ofsignal and interferers, successive interference canceling as well asspace-time or space-frequency filtering and other techniques. Moreinformation about these techniques can be found in the above-citedreferences by Theodore Rappaport and Paulraj, A. J., as well as otherpublications.

Interference mitigation including CCI reduction for the purpose ofincreasing spectral efficiency of cellular wireless systems particularlyadapted to a system using adaptive antenna arrays has been addressed inthe prior art. For example, U.S. Pat. No. 5,819,168 to Golden et al.examines the problem of insufficient estimation of CCI and noise incommunication channels which leads to an inability to suppressinterference. In particular, Golden teaches to solve the problemsassociated with correct estimation of the R_(xx) correlation matrix byan improved strategy for determining the weighting coefficients tomodify R_(xx) based on the ratio of interference to noise.

U.S. Pat. No. 5,933,768 to Sköld et al. addresses the problem ofinterference suppression with little knowledge of the interferingsignal. This is done by detecting a training sequence or other portionof the interfering signal, estimating the interferer channel and usingthis information in a joint demodulation receiver. The trainingsequences come from a finite set of known training sequences.Furthermore, the training sequences of the interferers arrive at thereceiver at undetermined times. The channel estimation is performed userby user and results in poor channel estimates of the interferers sincetheir training sequences can overlap the higher powered random datasequence of the desired user signal.

In yet another communication system as taught in U.S. Pat. No. 5,448,753to Ahl et al. interference is avoided. This is done by coordinating thedirection and transmission times of the beams such that they do notcross. In this manner interference between switched beams in a networkand especially between beams from adjacent base stations can be avoided.A significant effort has to be devoted to coordination between the usersand the base stations in this scheme.

Unfortunately, the above-discussed and other methods to improve spectralefficiency by CCI suppression in wireless systems including adaptiveantenna array systems do not exhibit sufficiently high performance.Thus, it would be desirable to improve interference suppression inwireless systems including systems using adaptive antenna arrays. Inparticular, it would be desirable to improve CCI suppression such that ahigher rate of frequency re-use could be employed in wireless systems.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea method to mitigate the effects of Co-Channel Interference (CCI) and awireless system adapted to practice this method.

It is a further object of the invention to provide for a sufficientlevel of CCI suppression to enable a higher frequency re-use in cellularwireless systems.

Yet another object of the invention is to adapt the method for use inwireless systems employing adaptive antenna arrays to further increaseCCI suppression performance.

The above objects and advantages, as well as numerous other improvementsattained by the method and apparatus of the invention are pointed outbelow.

SUMMARY

The objects and advantages of the invention are achieved by a method forinterference mitigation in a wireless communication system havingmultiple transmitters and receivers. In a first embodiment of themethod, at least a first transmitter and a second transmitter of thesystem transmit a first signal S₁ and a second signal S₂ respectivelyboth at a frequency f₁. One of the receivers located within a coveragearea receives first and second signals S₁, S₂. In accordance with themethod a time delay is determined between reception at a specific pointin the coverage area of the first and second signals S₁, S₂. Then, atransmission delay τ between the transmission of the first signal S₁ andthe transmission of the second signal S₂ is introduced such that signalsS₁, S₂ are received coherently at that specific point in the coveragearea. Because of that, signals S₁, S₂ are received substantiallycoherently or even coherently (when the point is at the location of thereceiver) by the receiver. This coherent reception aids in interferencemitigation.

The specific point in the coverage area can be located at the positionof the receiver and can be determined by ranging. Alternatively, thedistribution of the receivers in the coverage area is examined and theircenter of distribution is determined. The specific point in the coveragearea is substantially coincident with the center of the distribution.Frequently, this point will be located on an axis of symmetry of thecoverage area. For example, when the coverage area is a sector of acell, the point can be located on the axis of symmetry of that sector.

Now, when first signal S₁ is the useful signal and signal S₂ is aninterfering signal the method calls for estimating the channels ofsignals S₁, S₂ and applying a method of interference mitigation inrecovering signal S₁. Depending on the system, the method ofinterference mitigation can include beamforming, joint detection,successive interference canceling, space-time filtering, space-frequencyfiltering or any other suitable technique or combination.

To further aid in interference mitigation, it is preferable that signalsS₁, S₂ be assigned a first and a second training pattern respectively.The training patterns are chosen to be distinguishable by the receiver.Furthermore, the patterns are selected to optimize interferencemitigation. In some embodiments the patterns can also be adapted tosystem operating parameters such as communication traffic volume.Additionally, the training patterns can be selected based on a feedbackparameter, e.g., a measure of the quality of interference mitigation,obtained from the receiver.

The present method is preferably used in wireless communication systemswhich re-use frequencies such that the first and second transmitterstransmit signals at the same set of predetermined frequencies f₁, . . ., f_(n). The method can be used in bidirectional communications, e.g.,in the downlink and uplink.

A wireless system of the invention can re-use frequencies moreaggressively. For example, in the downlink the transmitters can be basestations in two cells located in close proximity or even adjacent eachother. The receiver can be a mobile or fixed wireless subscriber device.In the uplink the transmitters are typically wireless subscriber devicesand the receiver can be a base station. In either case the wirelesssubscriber devices and the base stations can use antenna arrays tofurther aid in interference mitigation in accordance with knowntechniques.

The base stations can be controlled by a base station control, as isknown. In one embodiment, the base station control is responsible forintroducing the transmission delay τ.

The method of the invention can be used in any cellular wireless systemwhich takes advantage of frequency re-use and seeks to reduce CCI. Themethod is particularly well-suited for use in systems which employantenna arrays in its transmitters and receivers for interferencemitigation. A wireless communication system employing the method of theinvention has a mechanism for determining a time delay between receptionof signals at the specific point in the coverage area. It also has acoordinating mechanism for introducing the transmission delay τ. Thewireless system can be a Time Division Multiple Access system (TDMA),Code Division Multiple Access (CDMA), Frequency Division Multiple Access(FDMA) or other multiplex communication systems using a multiple accessmethod or a combination of such methods.

The base station control or even the master station control of thewireless system have the necessary mechanisms or circuitry forperforming the functions called for by the method, such as thecoordinating mechanism for introducing transmission delay τ. In additionthe base station control can have a training unit for assigning thetraining patterns.

Preferably, an analyzer is provided for analyzing the interferencebetween the signals at the receiver. In fact, the analyzer is preferablya part of the receiver. In any event, it is preferable that the analyzerand the training unit are in communication and that the analyzergenerate a feedback parameter indicating a quality of interferencemitigation. This feedback is sent to the training unit which uses it inassigning training patterns.

In another method of the invention the training patterns are assigned tothe signals and the coordinated reception at the receiver is such thatthe training patterns are received coherently at the specific point inthe coverage area and substantially coherently by the receiver. Thismethod can be implemented in a wireless system equipped with a trainingunit for assigning the training patterns, as described above. A detaileddescription of the invention and the preferred and alternativeembodiments is presented below in reference to the attached drawingfigures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A (Prior Art) is a diagram showing a typical cluster of cells.

FIG. 1B (Prior Art) is a diagram of a wireless system composed of cellclusters as shown in FIG. 1A.

FIG. 2 is a diagram illustrating signal delay times in a number ofcells.

FIG. 3A&B are timing diagrams indicating appropriate transmission timesin the cells of FIG. 2.

FIG. 4 is a diagram illustrating a generalized wireless system utilizingthe method of the invention.

FIG. 5A is a timing diagram illustrating the signal transmission delaysused for coherent reception at the distribution center.

FIG. 5B is a timing diagram illustrating an acceptable delay inreceiving the signals at a mobile receiver.

FIG. 6 is a diagram of a wireless system utilizing the method of theinvention to increase frequency re-use.

FIG. 7 is a block diagram of a base station control for operating awireless network employing the method of the invention.

FIG. 8 is a block diagram of a base station of a cell from the networkof FIG. 7.

FIG. 9 is a block diagram of a subscriber unit with the requisiteelements for interferer cancellation.

FIG. 10A is a block diagram of a multi-channel estimator.

FIG. 10B is a block diagram of an interference suppression arrangement.

FIG. 11 is a block diagram of another embodiment of a channel estimatorand interference canceler.

DETAILED DESCRIPTION

The method of coordinated transmission to ensure substantially coherentreception of signals in accordance with the invention will be bestunderstood by first reviewing a portion of a wireless system 11 havingthree cells 13A, 13B, 13C with corresponding base stations 15A, 15B, 15Cas shown in FIG. 2. Base stations 15A, 15B, 15C transmit signals S₁, S₂,S₃ at the same frequency f₁ within sectors 17A, 17B, 17C. A receiver 32,which can be a fixed or mobile wireless device is shown in sector 17A.

For coherent reception of signals S₁, S₂, S₃ at receiver 32, propertransmission delays τ₀, τ₁, τ have to be introduced at base stations15A, 15B, 15C. Specifically, in the position shown receiver 32 is atdistances d′₀, d′₁ and d′₂ from base stations 15A, 15B, 15Crespectively. Thus, for coherent reception, transmission delays τ₀, τ₁,τ₂ are calculated based on those distances and introduced as shown inthe diagram of FIG. 3A.

In practice, receiver 32 is only one of a number of receivers (fixed ormobile) distributed throughout sector 17A. In fact, in a typicalsituation the distribution of receivers throughout sector 17A is uniformor nearly uniform. Sector 17A has an axis of symmetry 36 and for auniformly distributed set of receivers a center of the distribution C.D.lies on axis 36 at the geometrical center of sector 17A as shown. Thedistances from base stations 15A, 15B, 15C to C.D. are d₀, d₁ and d₂respectively. For coherent reception of signals S₁, S₂, S₃ at C.D. thetransmission delays which have to be introduced are shown in the diagramof FIG. 3B.

Arranging for the transmission delays to be such that coherent receptionis ensured at C.D. optimizes reception for all receivers in sector 17A.Of course, receivers closest to C.D. enjoy the highest receptioncoherence. All receivers receive signals S₁, S₂, S₃ substantiallycoherently or within a short time δ, as shown in FIG. 3B for receiver32. Of course, as the distribution of receivers changes, especially whenreceivers are all mobile receivers, C.D. will tend to move somewhat. Ifsufficient computational capacities are provided, then the movement ofC.D. can be tracked and taken into account to continuously maintain thebest reception coherence for the largest number of receivers.Additionally, the cells may not be symmetrical, as shown, and more thanjust three cells or rather signals from more than three base stationshave to be taken into account to achieve sufficient interferencemitigation.

In fact, a generalized wireless system 20 employing the method of theinvention in the downlink is shown in FIG. 4 to further clarify themethod of the invention. System 20 has a number of cells, 22A, 22B, . .. , 22X, 22Y within which radio coverage is provided by correspondingtransmitting units or base stations 24A, 24B, . . . , 24X, 24Y withrequisite transmission devices, e.g., antennas. Cell 22X in this exampleis a supercell, with its base station 24X antenna positioned at alocation providing line-of-sight communication for most signals. Forexample, base station 24X antenna can be placed on a mountain top or ona high building structure. Remaining cells are standard cells which aresubject to multi-path propagation of signals. Of course, any given cellcan have more than one base station, or it can consist of severalmicro-cells with independent re-transmission units in communication withthe base station. Also, the cells can be of different spatial extent.The base stations can in principle include any types of fixed or mobilebase stations, or ad hoc base stations. Alternatively, any of the basestations can be mounted on any suitable platform such as a satellite,terrestrial balloon, spaceship, etc. For simplicity these possibilitiesand corresponding adaptations are not shown in FIG. 4 but they will beapparent to a person skilled in the art.

Base stations 24A, 24B, . . . , 24X, 24Y can send out communicationsignals in various frequency channels centered at corresponding centerfrequencies (sometimes also referred to as sub-channels) within thebandwidth or spectrum assigned to system 20. For simplicity, thefrequency channels will be referred to herein by their centerfrequencies or just frequencies.

In system 20 base stations 24A, 24B, . . . , 24Y preferably use antennaarrays or directional antennas which transmit at frequency f₁ withinsectors 26A, 26B, . . . , 26Y. The remaining areas of cells 24A, 24B, .. . , 24Y may or may not be subdivided into sectors and can communicateat other frequencies which may or may not be re-used. Of course, not allantennas have to be directional, e.g., the antenna of base station 24Xis omnidirectional and communicates at f₁ within its entire coveragearea.

System 20 employs a frequency re-use scheme such that signals S₁, S₂, .. . , S_(x), S_(y) are transmitted at the same frequency f₁. In fact,each of these signals S₁, S₂, . . . , S_(x), S_(y) may itself representa group of useful signals, e.g., S_(1a), S_(1b), . . . etc. when singlebase multiplexing is enabled. A person of average skill in the art willrealize how to adopt the method and system of the instant invention insuch situations. For purposes of clarity, however, these multiplexingoptions are not explicitly discussed herein.

Depending on the cell and sector in which a signal is received, it iseither a useful signal or an interfering signal (interferer)contributing to CCI. In the simplest case, signal S₁ is a useful signalin sector 26A of cell 22A, but signals S₂, S₃, . . . , S_(x), S_(y) areall interfering signals in sector 26A of cell 22A. In general, however,any subset of signals S₁, S₂, . . . , S_(x), S_(y) can represent theuseful signal and the remaining subset of received signals can representthe interferers, as will be apparent to a person skilled in the artfamiliar with spatial multiplexing techniques. For example, spatialmultiplexing can be employed to provide communication of numeroussignals in the same allocated bandwidth as described in U.S. Pat. No.5,345,599. In the embodiment shown in FIG. 4 only one useful signal ineach sector is shown for the sake of clarity.

In the simple scenario discussed here, although attenuation ofelectromagnetic radiation provides for attenuation of signals S₂, S₃, .. . , S_(x), S_(y) according to the distance they propagate to reachcell 22A, any or all of these signals can arrive in sector 26A of cell22A by a direct or multi-path route at a sufficient signal strength torepresent CCI. The route or channel 28 of signal S₂ from sector 26B ofcell 22B to sector 26A of cell 22A is indicated in FIG. 4. Signals S₃,S₄, . . . , S_(y) also propagate to sector 26A in their respectivechannels (not shown).

A cellular user or subscriber 30 with wireless subscriber device 32 suchas a mobile, portable or stationary unit, in this case a mobile cellulartelephone operating in sector 26A receives all signals S₁, S₂, . . . ,S_(y) To device 32 signal S₁ is the useful signal and signals S₂, S₃, .. . , S_(y) are interferers. Interference can be mitigated by employingany suitable scheme such as, beamforming, joint detection, successiveinterference canceling, space-time filtering, space-frequency filteringor any other suitable technique or combination. For example, in thebeamforming method a received signal correlation matrix R_(xx) containsinformation of the routes or channels for each of the interferers S₂,S₃, . . . , S_(y) as well as the useful signal S₁. It is known that ifthe elements of the correlation matrix R_(xx) are known, i.e., if allchannels are known, then the channels carrying the undesired signals canbe canceled out. In the joint detection case the useful signal andinterferers are detected jointly in a similar fashion using knowledge ofthe channels. After detection the interfering signals are removed andthe useful signal(s) is kept.

Signals from cells closest to cell 22A, i.e., adjacent cells 22B, 22C,as well as cells directly aligned with cell 22A, e.g., cell 22D alongaxis 36 will be least attenuated. Hence, signals S₂, S₃, and S₄ willcontribute the most to CCI in sector 26A of cell 22A. Signals fromfurther away along axis 36, e.g., cell 22X and further laterally offsetfrom axis 36, e.g., cell 22Y will contribute less to CCI. In otherwords, signals S_(x), S_(y) will contribute less to CCI in sector 26A ofcell 22A. For best communication performance between base station 24Aand subscriber unit 32, however, any interferer received by unit 32should preferably be mitigated.

System 20 can use any suitable communication protocols for formattingthe data contained in signals S₁, S₂, . . . , S_(y) it transmits frombase stations 24A, 24B, . . . , 24Y. A base station control (BSC) 34 anda Master Station Control (MSC) 35 which controls BSC 34 and any otherBSCs (not shown) of system 20 control the transmission of signals S₁,S₂, . . . , S_(y) In accordance with the method, transmission delays areintroduced by base stations 24A, 24B, . . . , 24Y under supervision ofBSC 34 and/or MSC 35 such that all signals are received coherently atC.D. in sector 26A. This is shown in the diagram of FIG. 5A. As aresult, receiver 32 receives signals substantially coherently or withina time δ as shown in the diagram of FIG. 5B.

Substantially coherent reception of signals within time δ in and ofitself results in improved interference mitigation and reducedinter-symbol interference. It should be noted that guard intervals G ofsignals S₁, S₂, . . . , S_(y) should preferably be kept longer than δ,because of multi-path and other effects which can broaden time δ.

In a particularly advantageous embodiment of the method each signal S₁,S₂, . . . , S_(y) is additionally provided with a training pattern, orin this case a training sequence tr. The actual form of the trainingpattern or sequence will depend on the type of system 20 and signalcoding. In the case where each of the signals S₁, S₂, . . . , S_(y)represents a group of signals, each group will need multiple trainingpatterns. The type of training pattern for each constituent signal ineach of these groups will vary depending on the type of operation. Themultiple signals in each group could, for example, represent diversitystreams, multiplexing streams, etc. Multiplexing streams in particularmay need longer duration of training sequences (or patterns) than thoserequired for diversity streams to ensure similar accuracy of channelestimates. In case of single carrier modulation schemes, the trainingpattern can be, for example, a sequence of symbols or bits. But in caseof multi-carrier modulation schemes such as OFDM, the training patternmay comprise a set of frequency tones which are chosen out of theavailable tones in such a way that the individual tones in the set areorthogonal to each other. In this case the orthogonality between suchtwo different training patterns or frequency tone sets can be ensured bychoosing the correct constituent tones in those two sets. In the singlecarrier case, orthogonality between two training sequences depends onthe cross-correlation between them at the receiver. A person of averageskill in the art will realize what particular training patterns shouldbe used in any particular wireless system.

In the case shown in FIGS. 5A and 5B single carrier transmission isassumed and the training patterns are simple training sequences tr₁,tr₂, . . . tr_(y). Because of the coordinated transmission of signals,the training sequences are received substantially coherently by device32. In fact, thanks to the presence of guard intervals G, discussedbelow, the receipt of training sequences can be compensated for time δ.Such coherent reception of training patterns tr₁, tr₂, . . . tr_(y)further aids in interference mitigation. In this case, BSC 34 is incommunication with base stations 24A, 24B, . . . , 24Y of system 20 andwith subscriber units, such as subscriber unit 32 assign trainingsequences tr₁, tr₂, . . . , tr_(y) to signals S₁, S₂, S_(y). Thecomponents performing this assignment will be discussed below.

The time delays with which signals S₁, S₂, . . . , S_(y) arrive at C.D.are calculated using the known propagation speed (c=speed of light) ofthe electromagnetic signals and the known distances d₁, d₂, . . . ,d_(n). Of course, even if the base stations were mobile, these distancescan be periodically re-computed to determine the time delays e.g., byranging or other distance determination techniques known in the art. Infact, ranging from any base station or even subscriber unit 32 can beused at any point in time to re-confirm or determine distances d₁, d₂, .. . , d_(n). It should be noted that delay times between base stationscan be unequal if the cells of system 20 are not of the same size andthus the distances along axis 36 between successive base stations areunequal.

In the generalized case shown in FIG. 4 signals S₂ and S₃ experience atime delay τ₁ in propagating to base station 24A and another time delayτ₀ before being received at subscriber unit 32. For signal S_(y), whichis still sufficiently strong in sector 26A to interfere with signal S₁and requires CCI interference mitigation, the total time delay isτ+τ₁+τ₂+ . . . τ_(n). Clearly, it is only necessary to determine timedelays for signals which contribute to CCI.

For coherent reception of signals S₁, S₂, . . . , S_(y) and inparticular of their training sequences at subscriber unit 32, the time δhas to be taken into consideration. The guard intervals G₁, G₂, . . . ,G_(y) of duration at least equal to δ are added to signals to compensatefor time δ. The use of guard intervals or bits is well-known in the art.FIGS. 5A and 5B illustrate the formatting of signals in generalizedsystem 20 of FIG. 4. Specifically, signals S₁, S₂, . . . , S_(y) arebroken up into three main constituent portions, namely their guardintervals G₁, G₂, . . . , G_(y). their training sequences tr₁, tr₂, . .. , tr_(y) and their payload or data portions D₁, D₂, . . . , D_(y).

Because the total time of flight (TF) of signal S_(y) to unit 32 is thelongest, S_(y) is transmitted first at time t_(o). Signal S_(x) istransmitted after a transmission delay τ_(n) at which time signal S_(y)has already propagated distance d_(n) (see FIG. 4). In other words,S_(x) is transmitted at t₀+τ_(n) and approximately in sync with signalS_(y) indicated in dashed lines at t₀+τ_(n). After transmittingintervening signals (not shown) in the same manner, signals S₂, S₃ aretransmitted at time t₀+τ_(n)+ . . . +τ₁ and signal S₁ is finallytransmitted at time t₀+τ_(n)+ . . . +τ₁.

The above staggered transmission scheme or walking across scheme ensuresthat signals S₁, S₂, . . . , S_(y) are received coherently at time${TF} = {t_{o} + {\sum\limits_{i = 0}^{n}\quad \tau_{i}}}$

at C.D. and substantially coherently at unit 32, as indicated in dashedlines. More importantly, this scheme with additional compensationoffered by guard intervals G₁, G₂, . . . G_(y) ensures that trainingsequences tr₁, tr₂, . . . , tr_(y) are available to unit 32simultaneously.

In accordance with one embodiment of the invention, simultaneousreception of training sequences tr₁, tr₂, . . . , tr_(y) enables unit 32to determine the channels of signals S₁, S₂, . . . , S_(y) by obtainingaccurate channel estimates. In other words, unit 32 can now determinethe received signal correlation matrix R_(xx) and r_(xd) andsuccessfully use, e.g., the beamforming technique for interferencemitigation. Once matrix R_(xx) and r_(xd) are known, CCI can bemitigated. Of course, unit 32 can also use any of the other interferencemitigation techniques mentioned above.

It is known in the art that training patterns used will impact how wellthe channel of the corresponding signal can be determined. In general,longer training sequences or more dense training patterns will ensurebetter channel estimation. On the other hand, excessively long trainingsequences or dense training patterns take bandwidth away from thepayload. Thus, in general, training patterns should be chosen to yieldsufficiently good channel estimates for interference mitigation but notunduly limit the payload size.

The above generalized description illustrates the basic principles ofthe method and system of the invention. These principles can be adaptedto various wireless data transmission protocols and wireless systems.For example, the method of the invention can be used in a time divisionmultiple access (TDMA) network 50, a portion of which is shown in FIG.6. Because this system employs the method of invention for CCImitigation a more aggressive frequency re-use schedule is applied innetwork 50. In particular, the available spectrum is subdivided intoonly three sub-channels f₁, f₂, and f₃ which are re-used in threesectors of each cell 52 as shown. In the figure, cells 52 are regularlyspaced and of the same size, such that the distances between theircentrally positioned base stations are constant. Hence, the delay timesand the necessary transmission delays T are all equal. A person skilledin the art will recognize that in practice there will be deviations incell sizes and thus delay times may not be equal.

In order to ensure substantially coherent reception by the receivers ofsignals and/or their training sequences the base stations have tointroduce transmission delay τ in the manner described above. Forsignals transmitted at f₁ along a direction of orientation 56 of sectorsoperating at f₁ signals are transmitted from the most remote basestation row 54A which will produce CCI first at time t₀. Then, signalsare sent at time t₀+τ from the next row 54B of base stations which willproduce interference. Finally, at time t₀+2τ signals are sent from thelast row 54C of base stations which transmit useful signals tosubscriber units in the corresponding cells. Preferably, this staggeredtransmission or walking across network 50 scheme is performed across theentire network 50. The same walking across scheme is utilized intransmitting signals at frequencies f₂ and f₃.

FIG. 7 illustrates a Base Station Control (BSC) 60 and/or MasterSwitching Center (MSC) which can be used to control part of network 50.Only three cells 52A, 52B, 52C of network 50 are shown for clarity, butit is understood that BSC 60 and/or MSC as well as any additional BSCsare appropriately connected, as is known in the art, to control allcells 52 of network 50. In particular, BSC 60 is connected to basestations 53A, 53B, 53C of cells 52A, 52B, 52C.

BSC 60 has a training coordinator or controller 62 and a database oftraining patterns 64. Controller 62 is connected to database 64. The setof training patterns in database 64 can be different for differentre-use structures and cellular layouts while taking into account thechanging interference scenario. For example, training patterns can bedifferent time sequences such as Walsh codes in case of system 50 whichis a single carrier system. A person of average skill in the art willrecognize that other codes can be used depending on the type ofcommunication network. For example, different sets of frequency tonescan be used as training patterns in Orthogonal Frequency DivisionMultiplex (OFDM) systems. In fact, any orthogonal or other trainingpatterns which are distinguishable at the receiver and aid in effectivechannel estimation of the interferers can be employed.

BSC 60 communicates the selected training sequences to base stations53A, 53B, 53C through a signaling block 66. Base stations 53A, 53B, 53Cthen use these training sequences in the signals they transmit to thesubscriber units operating in their respective cells 52A, 52B, 52C. Forexample, training sequences for all frequencies f₁, f₂, f₃ used in cells52A, 52B, 52C are of the same length and are selected from the group ofWalsh code sequences.

Alternatively, the lengths and types of training sequences can beadjusted based on communication traffic volume. For instance, when notraffic exists in cell 53C in the sector operating at f₁ then no signalsare being transmitted from it and hence no signals from that sectorcontribute to CCI in any other cells, e.g. in the f₁ sector of cell 53A.Therefore, if system 50 is an OFDM system, then no training patterns arerequired by base station 53C for the f₁ sector, since no signals aretransmitted there. In other systems any training sequences can be keptshort and the training periods long. The bandwidth which would have beenallocated to the corresponding training sequence can thus be allocatedto training sequences used in other cells to allow more precise channelestimation or be used to increase the signal payloads in other cells.

On the other hand, when the traffic volume in the sector at f₁ in cell52C is high, its signals will have a major contribution to CCI in othercells, e.g., in the sector at f₁ in cell 52A. Hence, preferably a longtraining sequence is assigned by training controller 62 to the f₁ sectorin cell 52A to enable subscriber units in other cells to obtain asufficiently good estimate of these signals for interference mitigation.It is well-known to those skilled in the art that increasing the amountof training or the training sequence and decreasing the period oftraining, e.g., the times between training sequences, can improve theaccuracy of the channel estimate.

The performance of interference mitigation for any particular set ofassigned training sequences is preferably monitored. In this case BSC 60has a feedback analyzer 68 for receiving performance feedbackinformation from base stations 53A, 53B and 53C. Preferably, analyzer 68receives the signal quality feedback from the subscriber units throughtheir respective base stations, analyzes them and passes on the resultsto training controller 62. The monitoring can be performed continuouslyor periodically. The signal quality information can simply be a channelor link quality indication, such as individual signal strength, relativesignal strength amongst other signals or the mean square error of therespective channel estimate. Feedback analyzer's 68 report to trainingcontroller 62 on the link quality can be used for determiningre-assignment of training sequences by training controller 62 when thelink quality drops below an acceptable threshold.

Of course, subscriber units operating in network 50 have to be told whattraining sequences are used by the useful signal and the interferers sothat after coherently receiving the signals and training sequences theycan cancel out the interferers. For this purpose, the base stationscommunicate the training sequence assignments to the subscriber units.This can be accomplished as illustrated in FIG. 8 on the example of basestation 53A.

Base station 53A has a training distribution block 70, a transceiverunit 72 and a number of antennas 74A, . . . , 74X forming an adaptiveantenna array. Although such adaptive antenna arrays are preferred, themethod can also be employed in base station with a different antennaconfiguration or a single antenna system. However, as is known in theart, adaptive antenna arrays can use spatial-signature monitoring andprovide for additional interference mitigation and are hence preferredfor both base stations and subscriber units. Training distribution block70 receives the training sequence assignments from signaling block 66 ofBSC 60 and passes them on to transceiver unit 72. Transceiver unit 72communicates the training sequences of potential interferers and of itsown signal or signals to the subscriber units within cell 52A viaantennas 74.

Although downlink communication direction is being described at thispoint, in uplink communication, i.e., when subscriber units are thetransmitters and base stations the receivers, the situation is analogousbut reversed and it is the base stations which will mitigateinterference due to signals from subscriber units. Hence, base stations53A, 53B, 53C need to know the training sequences used by the subscriberunits. BSC 60 communicates to base stations 53A, 53B, 53C the trainingsequences to be assigned to the subscriber units and the base stationsuse these training sequences, received in a co-ordinated manneraccording to the method of the invention, to mitigate CCI. Because thepositions of the subscriber units are more likely to change, and willchange for mobile subscriber units, ranging between subscriber units andbase stations for the purpose of determination of distances andtransmission delays is required in the uplink. Whereas, for fixedsubscribers ranging once in a while, or at the time of intialinstallation may suffice.

FIG. 9 is a block diagram of a subscriber unit 80 equipped to operate ina wireless system of the invention, e.g., in network 50. Unit 80 has anadaptive antenna array 82 consisting of antennas 82A, . . . 82X forreceiving signals. An Rf/Down Conversion/Sampling circuit 84 processesthe signals received by array 82 and down-converts and samples them.After down-conversion and sampling the signals are applied to aninterference mitigation block 86 which regularly receives channelestimates of signals of interest and of interferers from a multi-channelestimator 88. A training block 90 which receives the training sequenceinformation from the base station, selects the correct training sequencefor each signal, e.g., signals of interest and the interferers, andsupplies it to multi-channel estimator 88.

Preferably, unit 80 has its own database of training sequences 92 usedin network 50 (e.g., mirroring those in database 64). In this way, thetraining sequences are locally available to unit 80. The trainingsequences can be updated or changed to reflect those in database 64 asinstructed by BSC 60. Multi-channel estimator 88 uses the trainingsequences from training block 90 and down-converted signals to estimatethe desired signal and interferer signal channels in parallel.Multi-channel estimator 88 is also connected to a signal qualitymeasurement block 94 which analyzes the channel estimates and inconjunction with multi-channel estimator 88 measures the signal qualityof the desired signals and interferers. This information is fed back tobe transmitted via the base stations to feedback analyzer 68 andtraining controller 62 of BSC 60. Analyzer 68 and training controller 62use signal quality feedback information to assess interferencemitigation performance and assign/re-assign training sequences asdescribed above.

An example of a multi-channel estimator 100 suitable for use asestimator 88 is shown in FIG. 10A. Multi-channel estimator 100 is aMulti Input Multi Output (MIMO) Space-Time channel estimator using theLeast Squares approximation. The selected training sequences for thedesired signals and interferers are delivered, e.g., from training block90, to a transfer matrix generation unit 102. Unit 102 produces thetransfer matrix T^(H)(T·T^(H))⁻¹, where T is the matrix of trainingsequences in which each row is a particular training sequence for adesired signal or interferer and T^(H) is the Hermetian transpose of T.

The transfer matrix is supplied from unit 102 to a matrix multiplier104. Matrix multiplier 104 also receives the actual signals with theirtraining sequences from the multiple channels. These signals includeuseful signals and interferers. Multiplier 104 multiplies the transfermatrix by the signals to obtain a joint MIMO channel estimate which ispassed on to interference mitigation such as block 86.

FIG. 10B shows an exemplary interference mitigation block 110 which canbe used as block 86. The channel estimate obtained from block 100 is fedto a weights computation block 112, which computes the weights for thereceived signals and delivers them to a space-time equalizer 114.Equalizer 114 can be a least squares (LS), zero forcing (ZF), minimummean square estimator (MMSE), an ML equalizer, a successive interferencecanceling type equalizer or any other kind of equalizer known to thoseskilled in the art. Equalizer 114 applies the weights from block 112 tothe received signals during the data phase or portion and thussuppresses the interfering signal or signals to obtain the desiredsignal or signals.

FIG. 11 shows yet another type of interference mitigation circuitry 120which implements an estimator 122 and an interference canceler 124 in aspace-frequency wireless system, e.g., an OFDM system. In this case thetraining patterns contained in the signals are particular frequency tonesets and their values. There can be a dedicated training phase duringwhich the training patterns are transmitted and a data phase duringwhich the data are transmitted. Alternatively, the training patterns canbe transmitted along with the data by allocating a dedicated subset ofdata tones to the training patterns.

During the training phase the channels 1 through n of the OFDM signalsare received and transformed to the frequency domain by fast Fouriertransform block (FFT) 126. The transformed signals are delivered to aMIMO space-frequency channel estimator 128. Estimator 128 is alsosupplied with the training patterns assigned to the desired signals andthe interferers. Using these inputs estimator 128 generates the jointchannel estimate, which it forwards to a space-frequency equalizer 132of interference canceler 124.

Equalizer 132 can be a least squares (LS), zero forcing (ZF), minimummean square estimator (MMSE), an ML equalizer, a successive interferencecanceling type equalizer or any other kind of equalizer known to thoseskilled in the art. During the data phase equalizer 132 receives OFDMsignals contained in channels 1 through n transformed to the frequencydomain by FFT block 130. Equalizer 132 uses the joint channel estimatesobtained from estimator 128 to suppress the interferers and generate thedesired signals at its output.

It will be clear to one skilled in the art that the above embodiment maybe altered in many ways without departing from the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

What is claimed is:
 1. A method of interference mitigation bycoordinated transmission in a wireless communication system having atleast a first transmitter, a second transmitter and a receiver, saidreceiver being located within a coverage area, said method comprisingthe following steps: a) determining a time delay between reception at apredetermined point in said coverage area of a first signal S₁transmitted from said first transmitter at a first frequency f₁ and asecond signal S₂ transmitted from said second transmitter at said firstfrequency f₁; b) introducing a transmission delay τ between thetransmission of said first signal S₁ and the transmission of said secondsignal S₂ such that said first signal S₁ and said second signal S₂ arereceived coherently at said predetermined point, whereby said firstsignal S₁ and said second signal S₂ are received substantiallycoherently by said receiver, thereby aiding in interference mitigation;wherein a number of receivers including said receiver are present insaid coverage area, said number of receivers having a center ofdistribution, said predetermined point substantially coinciding withsaid center of distribution.
 2. A method of interference mitigation bycoordinated transmission in a wireless communication system having atleast a first transmitter, a second transmitter and a receiver, saidreceiver being located within a coverage area, said method comprisingthe following steps: a) determining a time delay between reception at apredetermined point in said coverage area of a first signal S₁transmitted from said first transmitter at a first frequency f₁ and asecond signal S₂ transmitted from said second transmitter at said firstfrequency f₁; b) introducing a transmission delay τ between thetransmission of said first signal S₁ and the transmission of said secondsignal S₂ such that said first signal S₁ and said second signal S₂ arereceived coherently at said predetermined point, whereby said firstsignal S₁ and said second signal S₂ are received substantiallycoherently by said receiver, thereby aiding in interference mitigation;wherein said predetermined point is located along an axis of symmetry ofsaid coverage area.
 3. A method of interference mitigation bycoordinated transmission in a wireless communication system having atleast a first transmitter, a second transmitter and a receiver, saidreceiver being located within a coverage area, said method comprisingthe following steps: a) determining a time delay between reception at apredetermined point in said coverage area of a first signal S₁transmitted from said first transmitter at a first frequency f₁ and asecond signal S₂ transmitted from said second transmitter at said firstfrequency f₁; b) introducing a transmission delay τ between thetransmission of said first signal S₁ and the transmission of said secondsignal S₂ such that said first signal S₁ and said second signal S₂ arereceived coherently at said predetermined point, whereby said firstsignal S₁ and said second signal S₂ are received substantiallycoherently by said receiver, thereby aiding in interference mitigation;wherein said first signal S₁ is a useful signal and said second signalS₂ is an interfering signal and said method further comprises estimationof the channel of signal S₁ and of the channel of signal S₂, and ofapplying a method of interference mitigation in recovering said firstsignal S₁.
 4. The method of claim 3, wherein said method of interferencemitigation is selected from a group of methods consisting ofbeamforming, joint detection, successive interference canceling,space-time filtering and space-frequency filtering.
 5. The method ofclaim 3, further comprising the step of assigning a first trainingpattern to said first signal S₁ and a second training pattern to saidsecond signal S₂, said first training pattern and said second trainingpattern being distinguishable by said receiver.
 6. The method of claim5, wherein said first training pattern and said second training patternare selected to optimize interference mitigation.
 7. The method of claim5, wherein said first training pattern and said second training patternare selected based on a communication traffic volume.
 8. The method ofclaim 5, wherein said first training pattern and said second trainingpattern are selected based on a feedback parameter from said receiver.9. A method of interference mitigation by coordinated transmission in awireless communication system having at least a first transmitter, asecond transmitter and a receiver, said receiver being located within acoverage area, said method comprising the following steps: a)determining a time delay between reception at a predetermined point insaid coverage area of a first signal S₁ transmitted from said firsttransmitter at a first frequency f₁ and a second signal S₂ transmittedfrom said second transmitter at said first frequency f₁; b) introducinga transmission delay τ between the transmission of said first signal S₁and the transmission of said second signal S₂ such that said firstsignal S₁ and said second signal S₂ are received coherently at saidpredetermined point, whereby said first signal S₁ and said second signalS₂ are received substantially coherently by said receiver, therebyaiding in interference mitigation; wherein said wireless communicationsystem re-uses frequencies such that said first transmitter transmits atpredetermined frequencies f₁, . . . f_(n) and said second transmitteralso transmits at said predetermined frequencies f₁, . . . f_(n).
 10. Amethod of interference mitigation by coordinated transmission in awireless communication system having at least a first transmitter, asecond transmitter and a receiver, said receiver being located within acoverage area, said method comprising the following steps: a)determining a time delay between reception at a predetermined point insaid coverage area of a first signal S₁ transmitted from said firsttransmitter at a first frequency f₁ and a second signal S₂ transmittedfrom said second transmitter at said first frequency f₁; b) introducinga transmission delay τ between the transmission of said first signal S₁and the transmission of said second signal S₂ such that said firstsignal S₁ and said second signal S₂ are received coherently at saidpredetermined point, whereby said first signal S₁ and said second signalS₂ are received substantially coherently by said receiver, therebyaiding in interference mitigation; wherein said first transmitter is afirst base station located in a first cell and said second transmitteris a second base station located in a second cell in close proximity tosaid first cell and said first receiver is a wireless subscriber device.11. The method of claim 10, wherein said second cell is adjacent saidfirst cell.
 12. The method of claim 10, wherein said first base stationand said second base station are controlled by a base station controland said base station control introduces said transmission delay τ. 13.A method of interference mitigation by coordinated transmission in awireless communication system having at least a first transmitter, asecond transmitter and a receiver, said receiver being located within acoverage area, said method comprising the following steps: a)determining a time delay between reception at a predetermined point insaid coverage area of a first signal S₁ transmitted from said firsttransmitter at a first frequency f₁ and a second signal S₂ transmittedfrom said second transmitter at said first frequency f₁; b) introducinga transmission delay τ between the transmission of said first signal S₁and the transmission of said second signal S₂ such that said firstsignal S₁ and said second signal S₂ are received coherently at saidpredetermined point, whereby said first signal S₁ and said second signalS₂ are received substantially coherently by said receiver, therebyaiding in interference mitigation; wherein said first transmitter is afirst wireless subscriber device, said second transmitter is a secondwireless subscriber device, and said receiver is a first base stationlocated in a first cell.
 14. A method of interference mitigation bycoordinated transmission in a wireless communication system having atleast a first transmitter, a second transmitter and a receiver, saidreceiver being located within a coverage area, said method comprisingthe following steps: a) determining a time delay between reception at apredetermined point in said coverage area of a first signal S₁transmitted from said first transmitter at a first frequency f₁ and asecond signal S₂ transmitted from said second transmitter at said firstfrequency f₁; b) introducing a transmission delay τ between thetransmission of said first signal S₁ and the transmission of said secondsignal S₂ such that said first signal S₁ and said second signal S₂ arereceived coherently at said predetermined point, whereby said firstsignal S₁ and said second signal S₂ are received substantiallycoherently by said receiver, thereby aiding in interference mitigation;wherein said receiver, said first transmitter and said secondtransmitter have antenna arrays for interference mitigation.
 15. Awireless communication system comprising: a) at least a firsttransmitter and a second transmitter, said first transmittertransmitting a first signal S₁ at a first frequency f₁ and said secondtransmitter transmitting a second signal S₂ at said first frequency f₁;b) a receiver located in a coverage area for receiving said first signalS₁ and said second signal S₂; c) a means for determining a time delaybetween reception at a predetermined point in said coverage area of saidfirst signal S₁ and of said second signal S₂; and d) a coordinatingmeans for introducing a transmission delay τ between the transmission ofsaid first signal S₁ and the transmission of said second signal S₂ suchthat said first signal S₁ and said second signal S₂ are receivedcoherently at said predetermined point, whereby said first signal S₁ andsaid second signal S₂ are received substantially coherently by saidreceiver, thereby aiding in interference mitigation; further comprisingan analyzer for analyzing the interference between said first signal S₁and said second signal S₂ at said receiver.
 16. The wirelesscommunication system of claim 15, wherein said coordinating means is abase station control, and said receiver further comprises said analyzer.17. A wireless communication system comprising: a) at least a firsttransmitter and a second transmitter, said first transmittertransmitting a first signal S₁ at a first frequency f₁ and said secondtransmitter transmitting a second signal S₂ at said first frequency f₁;b) a receiver located in a coverage area for receiving said first signalS₁ and said second signal S₂; c) a means for determining a time delaybetween reception at a predetermined point in said coverage area of saidfirst signal S₁ and of said second signal S₂; and d) a coordinatingmeans for introducing a transmission delay τ between the transmission ofsaid first signal S₁ and the transmission of said second signal S₂ suchthat said first signal S₁ and said second signal S₂ are receivedcoherently at said predetermined point, whereby said first signal S₁ andsaid second signal S₂ are received substantially coherently by saidreceiver, thereby aiding in interference mitigation; further comprisinga training unit for assigning a first training pattern to said firstsignal S₁ and a second training pattern to said second signal S₂, saidfirst training pattern and said second training pattern beingdistinguishable by said receiver.
 18. The wireless communication systemof claim 17, further comprising an analyzer for analyzing theinterference between said first signal S₁ and said second signal S₂ atsaid receiver, said analyzer being in communication with said trainingunit.
 19. The wireless communication system of claim 18, wherein saidanalyzer is in feedback communication with said training unit, saidanalyzer generating a feedback parameter and said training unit usingsaid feedback parameter in assigning said first training pattern andsaid second training pattern.
 20. A wireless communication systemcomprising: a) at least a first transmitter and a second transmitter,said first transmitter transmitting a first signal S₁ at a firstfrequency f₁ and said second transmitter transmitting a second signal S₂at said first frequency f₁; b) a receiver located in a coverage area forreceiving said first signal S₁ and said second signal S₂; c) a means fordetermining a time delay between reception at a predetermined point insaid coverage area of said first signal S₁ and of said second signal S₂;and d) a coordinating means for introducing a transmission delay τbetween the transmission of said first signal S₁ and the transmission ofsaid second signal S₂ such that said first signal S₁ and said secondsignal S₂ are received coherently at said predetermined point, wherebysaid first signal S₁ and said second signal S₂ are receivedsubstantially coherently by said receiver, thereby aiding ininterference mitigation; wherein said first transmitter is a first basestation, said second transmitter is a second base station and saidreceiver is a wireless subscriber device.
 21. A wireless communicationsystem comprising: a) at least a first transmitter and a secondtransmitter, said first transmitter transmitting a first signal S₁ at afirst frequency f₁ and said second transmitter transmitting a secondsignal S₂ at said first frequency f₁; b) a receiver located in acoverage area for receiving said first signal S₁ and said second signalS₂; c) a means for determining a time delay between reception at apredetermined point in said coverage area of said first signal S₁ and ofsaid second signal S₂; and d) a coordinating means for introducing atransmission delay τ between the transmission of said first signal S₁and the transmission of said second signal S₂ such that said firstsignal S₁ and said second signal S₂ are received coherently at saidpredetermined point, whereby said first signal S₁ and said second signalS₂ are received substantially coherently by said receiver, therebyaiding in interference mitigation; wherein said first transmitter is afirst wireless subscriber unit, said second transmitter is a secondwireless subscriber unit, and said receiver is a base station.
 22. Amethod for interference mitigation in a wireless communication systemhaving at least a first transmitter, a second transmitter and areceiver, said receiver being located within a coverage area, saidmethod comprising the following steps: a) transmitting to said receivera first signal S₁ from said first transmitter at a first frequency f₁and a second signal S₂ from said second transmitter at said firstfrequency f₁; b) assigning to said first signal S₁ a first trainingpattern; c) assigning to said second signal S₂ a second trainingpattern; and d) coordinating the reception of said first trainingpattern and said second training pattern at said receiver, such thatsaid first training pattern and said second training pattern arereceived coherently at said predetermined point, whereby said firstpattern and said second pattern are received substantially coherently bysaid receiver, thereby aiding in interference mitigation.
 23. The methodof claim 22, wherein said predetermined point is located at the positionof said receiver.
 24. The method of claim 22, wherein said predeterminedpoint is determined by ranging.
 25. The method of claim 22, furthercomprising the step of indicating said training patterns.
 26. The methodof claim 22, wherein a number of receivers comprising said receiver arepresent in said coverage area, said number of receivers having a centerof distribution, said predetermined point substantially coinciding withsaid center of distribution.
 27. The method of claim 22, wherein saidpredetermined point is located along an axis of symmetry of saidcoverage area.
 28. The method of claim 22, wherein said coverage areacomprises a sector of a cell.
 29. The method of claim 22, wherein saidfirst signal S₁ is a useful signal and said second signal S₂ is aninterfering signal and said method further comprises estimation of thechannel of signal S₁ and of the channel of signal S₂ and of applying amethod of interference mitigation in recovering said first signal S₁.30. The method of claim 29, wherein said method of interferencemitigation is selected from a group of methods consisting ofbeamforming, joint detection, successive interference canceling,space-time filtering and space-frequency filtering.
 31. The method ofclaim 29, wherein said first training pattern and said second trainingpattern are selected to optimize interference mitigation.
 32. The methodof claim 29, wherein said first training pattern and said secondtraining pattern are selected based on a communication traffic volume.33. The method of claim 29, wherein said first training pattern and saidsecond training pattern are selected based on a feedback parameter fromsaid receiver.
 34. The method of claim 22, wherein said wirelesscommunication system re-uses frequencies such that said firsttransmitter transmits at predetermined frequencies f₁, . . . f_(n) andsaid second transmitter also transmits at said predetermined frequenciesf₁, . . . f_(n).
 35. The method of claim 22, wherein said firsttransmitter is a first base station located in a first cell and saidsecond transmitter is a second base station located in a second cell inclose proximity to said first cell and said first receiver is a wirelesssubscriber device.
 36. The method of claim 35, wherein said second cellis adjacent said first cell.
 37. The method of claim 35, wherein saidfirst base station and said second base station are controlled by a basestation control and said base station control performs said step ofcoordinating the reception of said first training pattern and saidsecond training pattern at said receiver.
 38. The method of claim 22,wherein said first transmitter is a first wireless subscriber device,said second transmitter is a second wireless subscriber device, and saidreceiver is a first base station located in a first cell.
 39. The methodof claim 22, wherein said receiver, said first transmitter and saidsecond transmitter have antenna arrays for interference mitigation. 40.A wireless communication system comprising: a) at least a firsttransmitter and a second transmitter, said first transmittertransmitting a first signal S₁ at a first frequency f₁ and said secondtransmitter transmitting a second signal S₂ at said first frequency f₁;b) a training unit for assigning to said first signal S₁ a firsttraining pattern and to said second signal S₂ a second training pattern;c) a receiver located in a coverage area for receiving said first signalS₁ and said second signal S₂; and d) a coordinating means forcoordinating the reception of said first training pattern and saidsecond training pattern at said receiver, such that said first trainingpattern and said second training pattern are received coherently at saidpredetermined point, whereby said first pattern and said second patternare received substantially coherently by said receiver, thereby aidingin interference mitigation.
 41. The wireless communication system ofclaim 40 selected from a group consisting of TDMA systems, CDMA systems,FDMA systems and OFDMA systems.
 42. The wireless communication system ofclaim 40, further comprising a means for indicating said trainingpatterns.
 43. The wireless communication system of claim 40, whereinsaid coordinating means is a base station control.
 44. The wirelesscommunication system of claim 40, further comprising an analyzer foranalyzing the interference between said first signal S₁ and said secondsignal S₂ at said receiver.
 45. The wireless communication system ofclaim 44, wherein said coordinating means is a base station control, andsaid receiver further comprises said analyzer.
 46. The wirelesscommunication system of claim 45, wherein said analyzer is in feedbackcommunication with said training unit, said analyzer generating afeedback parameter and said training unit using said feedback parameterin assigning said first training pattern and said second trainingpattern.
 47. The wireless communication system of claim 40, wherein saidfirst transmitter is a first base station, said second transmitter is asecond base station and said receiver is a wireless subscriber device.48. The wireless communication system of claim 40, wherein said firsttransmitter is a first wireless subscriber unit, said second transmitteris a second wireless subscriber unit, and said receiver is a basestation.